Our media partner "The Mercury News" reports police discovered a second semi-automatic assault rifle in the shooter's home.
The also found three magazines holding about 30 rounds each.
RELATED: Santino William Legan: What we know about Gilroy Garlic Festival shooting suspect
A search of the 19-year old's car turned up a passport and a "Gilroy Garlic Festival" pamphlet.
Officers also found a clown mask and an open bottle of Jack Daniels.
A motive for the attack that killed three people and wounded 16 others is still unclear.